Some Ramblings...........This project was built mostly from junkbox parts, very few were actually purchased specifically for the Sweeper. As such, component substitution is wide open. The enclosures for both the Sweeper and the VCO were recycled from other uses. I merely use air conditioning duct sealer tape to cover any holes that I can't incorporate into the design. Notice the liberal use of the tape in the VCO box, covering a rather large hole where a battery holder used to be. Likewise, there's lots of room in the VCO enclosure and none in the sweep generator. For the front and rear panel lettering, I use printable sheets of adhesive-backed 8 1/2 x 11 paper, available at any office supply store. The panels are laid out using Windows 'Paint' program, with many test-prints on plain paper until the dimensions are correct. Then print the layout on the adhesive paper, cut to size and carefully apply to the enclosure, before any parts are mounted. If done on a color printer, a nice front panel can be made. For further protection, a clear adhesive sheet can be applied over the panel.
The use of an optical isolator is not necessary for the marker output. I used it because it was very simple and I don't like to mix negative supply voltages with 5 volt logic. Transistors don't usually open, they short!
If the 'Spot' pushbutton is held on, the VCO frequency can be varied between the setup points with the 'Marker' pot. This can be handy if you don't really care about setting an exact start and end frequency, but just want to see the response at a certain frequency.
If you are sweeping an unknown circuit or item, such as an unmarked crystal filter or resonator, start with the widest sweep possible. Once the peak is located on the display, the 'Min' and 'Max' pots may be used, without the 'Min/Max' switch, to center and widen the peak for detailed measurements.
To adjust the sweep width displayed on the oscilloscope, select ‘External Horizontal Input’ and use the ‘Horizontal volts / division’ and fine adjustment knob to display the desired sweep. The terminology used for these controls on various scopes may be different, but should be very close. In any case, the amplitude of the sweep signal determines the sweep width. The controls will probable allow for a wide range of sweep adjustment, from perhaps a ½ inch sweep, to a scan that extends well beyond both edges of the oscilloscope screen. Here lies another way to expand the scope display. By expanding the sweep beyond the physical limits of the screen and centering the display of interest, this widens the displayed peak, although the marker will also extend beyond the scope screen. The start and end VCO frequencies remain the same, but a smaller portion of the sweep is displayed, effectively decreasing the ‘frequency per division’.
As the sweep speed is increased, by increasing U2’s clock frequency, the displayed information becomes less accurate. As on a spectrum analyzer, increasing the sweep speed tends to act as a low pass filter on the display. This is because the scope trace is scanning so fast that the screen persistence won’t allow display of sharp peaks and valleys. Likewise, a very slow sweep rate will display every little nuance in the signal, but measurements are more difficult because of the opposite persistence effect ; it disappeared! I included the four position DIP switch to select the best sweep speed for whatever type of oscilloscope is used, and once set, it does not need to be adjusted further.
A signal that I was going to bring out to the rear panel of the sweeper is the U2 carry output, RCO on pin 13. This is a low signal, one clock pulse wide, that occurs at the beginning of each sweep. It could be used as a trigger to sync other equipment to the sweeper. It is not used on the prototype.
The prototype was built on several small printed circuit boards. More often than not, I find that it is just as easy and quick to make a pcb, as it is to wirewrap or perfboard a circuit using IC’s. The boards are hand-drawn, and drilled with a handheld Dremel tool. They are not fancy, but functional. Warmed-up Radio Shack ‘PCB Etchant Solution’ will etch a board in about ½ hour. First I lay the board out on .100 inch graph paper that I make on my computer using Microsoft Excel, or it can be purchased at an office supply store. Then tape the drawn pattern to the blank PCB. I drill the board next and smooth and clean it with a pot-scrubber, like ‘ScotchBrite’. I then use a ‘Sharpie’ or freezer-marking pen, available in most any grocery store, to draw the traces on the blank board. Then I etch the board, clean it once more with the scrubber, and it’s done. Dispose of the used etchant down the toilet, and don’t splash it anywhere. It will permanently stain anything it touches.
Finally, I just found that Mini-Circuits (www.mini-circuits.com), sells a neat ‘designers kit’ of seven DIP-sized VCO modules that cover the frequency range of 25 MHz to 1.025 GHz. They run on 12 volts and use a tuning voltage of 1 to 16 volts. Digi-Key sells a small, inexpensive DC to DC converter SIP, made by Power One, which runs off 5 volts, to take care of the increased tuning voltage. The VCO kit is part number K-POS2 and costs $79.95. If I decide to build something up, I will put all the details on this webpage.
Email me with any questions, comments or ideas at firstname.lastname@example.org.
Wayne Maxwell KD4YGU
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